The present invention generally relates to an implantable cardiac stimulation device. The present invention more particularly relates to a device and method for use with such a device for monitoring displacement of an implanted cardiac lead.
Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads, which are implanted within the heart. The electrodes are positioned within the right side of the heart, either the right ventricle or right atrium, or both, for making electrical contact with their respective heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired therapy.
Traditionally, therapy delivery had been limited to the right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. If a blood clot were released from the heart""s left side, as for example the left ventricle, it could pass directly to the brain potentially resulting in a paralyzing or fatal stroke. However, a blood clot released from the right heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain.
Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy from or to the left heart. These lead structures and methods avoid direct electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus, the great vein, or a lateral vein of the heart. As is well known, the coronary sinus passes closely adjacent the left atrium, extends into the great vein adjacent the left ventricle and then continues adjacent the left ventricle towards the apex of the heart via a lateral vein or the great cardiac vein.
It has been demonstrated that electrodes placed in the coronary sinus and a left ventricular vein may be used for left atrial pacing, left ventricular pacing, and cardioversion and defibrillation. These advancements enable implantable cardiac stimulation devices to address the needs of a patient population with left ventricular dysfunction and congestive heart failure which would benefit from left heart side pacing, either alone or in conjunction with right heart side pacing (multi-chamber pacing).
Since the benefit of multi-chamber pacing (bi-ventricular pacing or bi-atrial pacing) is dependent upon appropriate chamber synchronization and/or appropriate activation sequencing, the maintenance of proper placement of the pacing electrodes to pace the left side of the heart is paramount. Thus, it is desirable for an implantable cardiac stimulation device to have a system, which can identify if the left heart lead placement is unstable or has shifted.
The present invention therefore provides an implantable cardiac stimulation device and method for monitoring displacement of a cardiac lead implanted in a heart, such as a cardiac lead implanted in the left heart. In accordance with the present invention, a template is generated representing the impedance of the lead over a discrete time period, such as a cardiac cycle of the heart. Thereafter, at spaced apart times, subsequent lead impedance measurement signals are generated and compared to the lead impedance measurement template from which a comparison factor is derived. The comparison factors are indicative of relative displacement of the implanted cardiac lead.
The comparison factors may be stored in a memory over time and conveyed by a telemetry circuit to a non-implanted receiver for analysis. The comparison factors may be generated by employing template matching or feature extraction techniques.
Either all of the comparison factors or selected ones of the comparison factors may be stored in the memory. The selected ones of the comparison factors to be stored may be those factors, which have a value greater than a predetermined factor.
When a comparison factor is stored in memory, other data may also be stored along with the comparison factor. Such data may include the date and time of the comparison, the patient""s heart rate, the device mode, the posture of the patient, and/or portions of the digitized impedance waveforms. If the comparison factor is greater than the predetermined factor, a further template may be generated. Subsequent comparisons may then be made to all of the generated templates and corresponding comparison factors may be generated. The foregoing provides comprehensive data for the physician to track displacement of the implanted lead should displacement of the lead occur.